Desulfurisation of fuel

ABSTRACT

A method of removing sulfur from a fuel supply stream for a fuel cell ( 1 ), which method comprises: (a) hydrogenating the fuel supply stream ( 1 ) by contacting it with a hydrogenation catalyst in the presence of hydrogen to convert sulfur-containing compounds in the fuel supply stream ( 1 ) into hydrogen sulfide; (b) removing the hydrogen sulfide to produce a desulfurised fuel stream; and (c) pre-reforming the desulfurised fuel stream to produce a fuel cell feed stream, wherein a portion of the fuel cell feed stream is processed to increase its hydrogen content to produce a hydrogen-enriched fuel stream which is used in step (a) as a source of hydrogen to hydrogenate the fuel supply stream ( 1 ).

The present invention relates to treatment of a fuel supply for a fuelcell electrical power generating system and, in particular, to theremoval of sulfur from the fuel supply.

Fuel cells convert gaseous fuels via an electrochemical process directlyinto electricity. Typically, the fuel used contains sulfur in the formof hydrogen sulfide and organic sulfur-containing compounds such asmercaptans, and it is important to remove the sulfur from the fuel toavoid poisoning of catalysts used downstream in systems of the fuelcell. Conventional desulfurisation systems include hydrodesulfurisersconsisting of a hydrogenation catalyst to convert the sulfur-containingcompounds to hydrogen sulfide and a hydrogen sulfide absorbent bed. Thehydrogenation catalyst of the hydrodesulfuriser requires a continuoussupply of hydrogen in order to effect conversion of thesulfur-containing compounds to hydrogen sulfide.

There is a continuing desire to improve the overall efficiency of fuelcell systems and the present invention seeks to do so by innovation inthe way in which sulfur is removed from fuel for such systems.

Accordingly, the present invention provides a method of removing sulfurfrom a fuel supply stream for a fuel cell, which method comprises:

-   (a) hydrogenating the fuel supply stream by contacting it with a    hydrogenation catalyst in the presence of hydrogen to convert    sulfur-containing compounds in the fuel supply stream into hydrogen    sulfide;-   (b) removing the hydrogen sulfide to produce a desulfurised fuel    stream; and-   (c) pre-reforming the desulfurised fuel stream to produce a fuel    cell feed stream,    wherein a portion of the fuel cell feed stream is processed to    increase its hydrogen content to produce a hydrogen-enriched fuel    stream which is used in step (a) as a source of hydrogen to    hydrogenate the fuel supply stream.

The crux of the present invention resides in producing ahydrogen-enriched fuel stream and using this fuel stream to supplyhydrogen for hydrogenation of the initial fuel supply stream. In otherwords, the present invention is based on internal hydrogen generationand, in particular, hydrogen intensification to produce the hydrogenrequired to effect hydrodesulfurisation of the fuel supply stream.

The present invention also provides a fuel processing system useful forcarrying out the method described above. The system comprises:

-   -   hydrogenation means comprising a hydrogenation catalyst;    -   hydrogen sulfide removal means provided downstream from and in        communication with the hydrogenation means;    -   a pre-reformer provided downstream from and in communication        with the hydrogen sulfide removal means; and    -   hydrogen intensifier means which is provided downstream from and        in communication with the pre-reformer and upstream from and in        communication with the hydrogenation means, wherein the hydrogen        intensifier means receives a portion of the output from the        pre-reformer.

It will be appreciated from the foregoing that the hydrogen intensifiermeans is provided as part of a loop which returns a portion of theoutput stream of the pre-reformer (denoted the “fuel cell feed stream”herein) to the input stream (denoted the “fuel supply stream” herein) ofthe hydrogenation means. A portion of the output stream which is notreturned to the hydrogenation means is delivered to (the anode of) thefuel cell.

The present invention also provides a fuel cell system comprising a fuelprocessing system as described above and a fuel cell provided downstreamfrom and in communication with the pre-reformer. The fuel cell uses aportion of the fuel cell feed stream as fuel.

Unless the context otherwise permits, herein reference to a fuel cellmeans a fuel cell which is capable of internally reforming methane tohydrogen at the anode of the cell. The electrochemical electricitygenerating reactions within the cell are exothermic whereas the internalreforming reaction is endothermic. The amount of methane delivered tothe anode may therefore be controlled in order to achieve thermalmanagement of the fuel cell, depending upon the load requirements of thefuel cell. The concentration of methane delivered to the anode of thefuel cell may be manipulated by varying the temperature at which thepre-reformer is operated. Under conditions of high load, the temperatureof the fuel cell will increase and a relatively high concentration ofmethane is required for internal reforming on the anode to achievegreater cooling. In this case the pre-reformer would be operated atrelatively low temperature. The opposite is true when the fuel cell isoperating under low load. The kind of conditions under which thepre-reformer would operate are discussed in more detail below.

It will be appreciated from the foregoing that it is essential inaccordance with the present invention that a portion of the fuel cellfeed stream is processed in order to increase its hydrogen content and aportion of the same stream is delivered to (the anode of) the fuel cell,i.e the fuel cell feed stream from the pre-reformer is split. Productionof a feed stream enriched in hydrogen is unnecessary when using a fuelcell the anode of which is capable of internal reforming of methane.Splitting of the fuel cell feed stream affords generation of a hydrogenenriched stream for use in the hydrogenation means whilst retaining asuitable feed stream for internal reforming of methane at the anode ofthe fuel cell. The present invention is therefore fundamentallydifferent from systems in which the fuel cell does not have internalreforming capability and in which all of the output of the pre-reformeris processed to increase its hydrogen content followed by division ofthe resultant hydrogen rich stream between upstream hydrogenation meansand downstream fuel cell anode.

The various components of the fuel processing and fuel cell systemsdescribed herein are in communication with each other by means ofconventional gas supply conduits. These may also include ancillarycomponents such as heat exchangers, control valves, manifolds, pumps andcondensers, as necessary. The terms “upstream” and “downstream” areintended to reflect the positions of the various functional means of thesystems relative to each other. The accompanying figure also illustratesthis.

The fuel supply stream may comprise any (sulfur-containing) fuel whichis typically used in fuel cell systems and which may be pre-reformed togenerate methane. Thus, the fuel may be a higher (C₂₊) hydrocarbon fuelsuch as gasoline, diesel, kerosene, naphtha or LPG. Typically, the fuelsupply stream will be natural gas. This is predominantly methane withsmall quantities of higher hydrocarbons.

In accordance with the present invention, in a first stage, the fuelsupply stream is hydrogenated by contacting the stream with ahydrogenation catalyst in the presence of hydrogen. This may be achievedusing conventional equipment and processing conditions. Conventionalhydrogenation catalysts such a Co—Mo catalysts may be used. Prior tohydrogenation the fuel supply stream is typically heated using apre-heater so that the input stream for hydrogenation is at a suitabletemperature for the hydrogenation catalyst being used. When using Co—Mothe input stream may have a temperature of approximately 400° C.Hydrogenation results in conversion of sulfur-containing compoundspresent in the fuel supply stream into hydrogen sulfide and sulfur-freehydrocarbons.

Subsequent to hydrogenation the hydrogen sulfide is removed to produce adesulfurised fuel stream. This removal is typically achieved bycontacting with a hydrogen sulfide absorbent bed. Conventional hydrogensulphide absorbents may be used, for example ZnO. When ZnO is used ZnSis formed according to the reaction:H₂S+ZnO→ZnS+H₂OContinued reaction leads to consumption of the absorbent so that it mustbe changed periodically. The sulfur in its absorbed form may bediscarded or used for further chemical processing absorb.

Hydrodesulfurisation results in a desulfurised fuel stream which is thendelivered to a pre-reformer. Prior to delivery to the pre-reformer thesulfur content of the fuel is typically reduced to a level of less thanabout 1 part per million by weight, and preferably to less than 0.2parts per million by weight.

The desulfurised fuel stream is then pre-reformed and this may takeplace in a steam pre-pre-reformer. In the present invention thedesulfurised fuel stream is not usually fully reformed. The primaryfunction of the pre-reformer is to remove higher hydrocarbons andproduce a methane and steam rich stream, with varying levels of methanedepending upon the operating temperature of the pre-former and based onthe load requirements of the fuel cell as discussed above. Thepre-reforming operation may be carried out in conventional manner. Steampre-reforming is conveniently performed at atmospheric pressure, buthigher pressures may be adopted if desired, for example up to about 1000kPa. Steam pre-reforming is usually performed at a temperature nogreater than 450° C., more preferably in a range of about 250-450° C.and, dependent upon the fuel and other process parameters, most usuallyin a range of about 300-400° C. Under low load the temperature is likelyto be increased up to 600° C. In the pre-reformer higher hydrocarbonsare reformed to form methane, carbon monoxide, carbon dioxide andhydrogen.

Generally, the steam pre-reforming process will be carried out such thatthe higher hydrocarbon fuel is resident over the pre-reforming catalystfor a sufficient time to ensure at least substantially completeconversion of the higher hydrocarbons. This alleviates deposition ofcarbon on the anode in the downstream fuel cell when hydrocarbons arereformed on the anode. However, some higher hydrocarbons may be presentin the output fuel stream and preferably there is 97.5% or greaterconversion of hydrocarbons in the steam pre-reforming process. Morepreferably, there is no more than about 0.1% by volume higherhydrocarbons present in the fuel stream to the anode measured on a wetbasis.

A variety of conventional steam pre-reformers are known and any of thesemay be used. The common pre-reformer catalyst is nickel-based, but maycomprise, for example, platinum, rhodium, other precious metals or amixture of any of these.

A portion of the pre-reformer output (the “fuel cell feed stream”) isused to supply hydrogen to the hydrodesulfuriser operation. The key tothis step is hydrogen intensification that portion of the fuel cell feedstream. This is achieved by feeding a portion of the fuel cell feedstream to a hydrogen intensifier where further reforming of the streamtakes place resulting in increased hydrogen content. The hydrogenintensifier contains any conventional commercially available reformingcatalyst and this typically operates at elevated temperature (generallyfrom 500-600° C. but quite possibly higher, for example up to about 850°C.). To maintain the hydrogen intensifier at the requisite temperature,an electrical heating band may be used. Following hydrogenintensification the hydrogen-enriched fuel stream typically has ahydrogen content of 30 to 60%, more preferably of 50 to 60%, by volume.

The hydrogen-enriched stream is then preferably cooled and excess waterremoved. This has the advantage of further intensifying the hydrogenconcentration of the stream and of reducing the impact of moisture onthe effectiveness of the hydrogen sulfide absorbent. Cooling of thestream also enables a low temperature pump or recycle blower to be used.Typically, cooling is achieved using a conventional condenser unitprovided with a condensate trap. A low temperature pump or recycleblower, typically operating at a fixed rate, may be used to recirculatethe hydrogen-enriched fuel stream as an input of the hydrodesulfuriseroperation. The hydrogen-enriched fuel stream is usually mixed with thefuel supply stream prior to hydrogenation thereof. Alternatively, thehydrogen-enriched fuel stream may be fed directly to the hydrogenationcatalyst as a separate input to the fuel supply stream.

The accompanying non-limiting figure illustrates an embodiment of thepresent invention. In the figure a fuel supply stream (1), such asnatural gas, is fed via a fuel pre-heater (2) to a hydrodesulfuriserunit (3,4). The hydrodesulfuriser unit includes a hydrogenation unit (3)provided with a hydrogenation catalyst (such as a Co—Mo catalyst) and,downstream, a hydrogen sulfide absorbent bed (4), such as zinc oxide,operating at around 400° C. In the figure the hydrogenation unit (3) andhydrogen sulfide absorbent bed (4) are shown as separate components ofthe system. In practice these components may be combined in a singlevessel by mixing or staging of the hydrogenation catalyst and absorbent.Following hydrodesulfurisation a desulfurised fuel stream is fed to apre-reformer (5). At full load the pre-reformer may be operated in anadiabatic manner and under low load conditions electrical heating, orthe like, may be employed. A steam generator (6) fed with demineralisedwater (6 a) provides steam to the pre-reformer (5). Followingpre-reformation a fuel cell feed stream is produced, a portion of whichis passed to the anodes of a fuel cell stack (7). The anodes are capableof reforming methane to hydrogen. A portion of the fuel cell feed streamis tapped off and fed to a hydrogen intensifier (8) operating at atemperature of around 550° C. The output of the hydrogen intensifier isthen cooled in a condenser (9) and condensate (10) removed. Thehydrogen-enriched fuel stream produced is recirculated by a hydrogenrecycle blower (11) and mixed with the fuel supply stream (1) prior tothe latter being fed to the hydrodesulfuriser unit (3,4) where thehydrogen in the hydrogen-enriched fuel stream effects hydrogenation ofthe fuel supply stream (1) in the hydrogenation unit (3). Thehydrogen-enriched fuel stream may also be recirculated using ejectors orby connection upstream of a fuel pressure booster. Conceivably, if theintensifier is run at high temperature and an ejector or booster isused, the condenser (9) and or the recycle blower (11) may be disposedof.

On start up there will not be any pre-reformer output so that there willnot be any hydrogen available for the hydrodesulfurisation operation.The level of hydrogen in the fuel supply stream is likely to beinsufficient to provide effective hydrogenation. Initially at least,this means that hydrodesulfurisation of the fuel supply stream isinefficient when compared with when the pre-reformer output isrecirculated (with reformation and hydrogen intensification) to thehydrodesulfurisation operation. In practice, the system of the presentinvention may be designed to take this lag in pre-reformer output intoaccount. The quantity of the catalysts used downstream in the fuelprocessing and fuel cell systems are typically chosen to take intoaccount that there may be some sulfur slippage and catalyst degradationon start up. Thus, the pre-reformer catalyst may be oversized.Alternatively, a cold desulfuriser such as activated carbon may be usedon start-up. Once started it is envisaged that the fuel processingsystem will be run continuously to avoid recurrence of the start upproblem. Typically, the system will be run under low load/idling load asopposed to being shut down.

Typically, the concentration of hydrogen returned to the fuel supplystream to effect hydrodesulfurisation is in excess of that actuallyrequired based on the sulfur content of the fuel. The hydrogenconcentration of the stream returned to the hydrodesulfurisationoperation may be controlled and adjusted, for instance to suit thequality and type of the fuel supply stream. The hydrogen concentrationmay be manipulated by adjusting the temperature at which the hydrogenintensifier (reformer) is operated or by varying the rate of its supplyfrom the fuel cell feed stream. For instance, in the type of systemillustrated in the accompanying figure, when the operating temperatureof the hydrogen intensifier is 550° C. and 600° C. respectively, for afeed input hydrogen content of 7.5 to 13%, the hydrogen content in theoutput from the hydrogen intensifier will be around 37% and 47%respectively, which after condensation gives a hydrogen content ofaround 53% and 58% respectively. Thus, the temperature of the hydrogenintensifier/reformer may be used to manipulate the hydrogen content inthe output stream, and this may be further enhanced by condensation toremove water vapour.

The fuel cell used in practice of the present invention is typically asolid oxide fuel cell (SOFC) in which the fuel cell anodes are capableof internally reforming methane in order to generate hydrogen and carbonmonoxide. SOFCs tends to be regarded as the most efficient and versatilepower generation systems, in particular for dispersed power generation,with low pollution, high efficiency, high power density and fuelflexibility. SOFCs operate at elevated temperatures, for example700-1000° C. Other fuel cells which operate at elevated temperaturesinclude the molten carbonate fuel cell requiring a minimum temperatureof 650° C. However, SOFCs are the primary interest for the invention anddiscussion herein will be mainly directed to these without intending tobe limited in any way.

Numerous SOFC configurations exist, including the tubular, themonolithic and the planar design. Single planar SOFCs are connected viainterconnects or gas separators to form multi-cell units, sometimestermed fuel cell stacks. Gas flow paths are provided between the gasseparators and respective electrodes, for example by providing gas flowchannels in the gas separators. In a fuel cell stack thecomponents—electrolyte/electrode laminates and gas separator plates arefabricated individually and then stacked together. With thisarrangement, external and internal co-flow, counter-flow and cross-flowmanifolding options are possible of the gaseous fuel and oxidant.

Preferably, the anode in the fuel cell comprises a nickel material, suchas a nickel/zirconia cermet, which is used to catalyse the reformingreaction in the fuel cell. The fuel cell and its associated assembly cantake any suitable form provided it operates at a temperature of at least650° C. to provide at least substantial conversion of the methane in theinternal reforming reaction. By way of example only, several differentplanar SOFC components and systems, SOFCs and materials are described inour International Patent Applications PCT/AU/96/00140, PCT/AU96/00594,PCT/AU98/00437, PCT/AU98/00719 and PCT/AU98/00956, the contents of whichare incorporated herein by reference, including the corresponding USnational phase patent U.S. Pat. No. 5,942,349 and patent applicationSer. Nos. 09/155,061, 09/445,735, 09/486,501 and 09/554,709,respectively. Other disclosures appear in our International patentapplications PCT/AU99/01140, PCT/AU00/00630 and PCT/AU00/00631.

Generally, the fuel cell to which the fuel stream is supplied will beone of multiple fuel cells to which the fuel stream is also supplied,commonly called a fuel cell stack in the case of planar SOFCs, and it isenvisaged that a fuel cell stack will be employed in practice of thepresent invention. However, the invention also extends to the processbeing performed using a single fuel cell.

1. A method of removing sulfur from a fuel supply stream for a fuelcell, which method comprises: (a) hydrogenating the fuel supply streamby contacting it with a hydrogenation catalyst in the presence ofhydrogen to convert sulfur-containing compounds in the fuel supplystream into hydrogen sulfide; (b) removing the hydrogen sulfide toproduce a desulfurised fuel stream; and (c) pre-reforming thedesulfurised fuel stream to produce a fuel cell feed stream, wherein aportion of the fuel cell feed stream is processed to increase itshydrogen content to produce a hydrogen-enriched fuel stream which isused in step (a) as a source of hydrogen to hydrogenate the fuel supplystream.
 2. A method according to claim 1, wherein the fuel supply streamis gasoline, diesel, kerosene, naphtha, LPG or natural gas.
 3. A methodaccording to claim 1, wherein the desulfurised fuel stream ispre-reformed in a steam pre-reformer.
 4. A method according to claim 1,wherein the hydrogen content of a portion of the fuel cell feed streamis increased by feeding the fuel cell feed stream to a hydrogenintensifier where further reforming of the stream takes place resultingin increased hydrogen content.
 5. A method according to claim 4, whereinthe hydrogen-enriched stream is then cooled and excess water removed. 6.A method according to claim 4, wherein the desulfurised fuel stream ispre-reformed in a steam pre-reformer and the hydrogen intensifier is areformer operating at a higher temperature than the steam pre-reformer.7. A fuel processing system comprising: hydrogenation means comprising ahydrogenation catalyst; hydrogen sulfide removal means provideddownstream from and in communication with the hydrogenation means; apre-reformer provided downstream from and in communication with thehydrogen sulfide removal means; and hydrogen intensifier means which isprovided downstream from and in communication with the pre-reformer andupstream from and in communication with the hydrogenation means, whereinthe hydrogen intensifier means receives a portion of the output of thepre-reformer.
 8. A system according to claim 7, wherein the pre-reformeris a steam pre-reformer.
 9. A fuel cell system comprising a fuelprocessing system claimed in claim 7 and a fuel cell provided downstreamfrom and in communication with the pre-reformer, the anode of the fuelcell receiving a portion of the output of the pre-reformer.